Receptive areas of primary infrared afferent neurons in crotaline snakes

Using crotaline snakes, extracellular recordings were made from trigeminal ganglion cells which responded to infrared stimulation. Eighty infrared receptive areas of primary afferents were delineated by scanning a Hc-Ne laser spot across the pit membrane of eleven snakes. There was only one receptive area for each neuron. The receptive areas were roughly round in shape, with diameters of 20–50μm. Sensitivity was 10 −1−10 −3 of the unattenuated laser intensity (160W/cm 2 at a wavelength of 632.8 nm). There were no inhibitory areas or cold-sensitive areas on the pit membrane, and there were no areas with directional selectivity. Since the measured receptive areas were comparable in size with reported histological data (receptors are about 40 μm in dia.), and since one unit had only one receptive area, we exclude the hypothesis of fibre branching. Comparison of background discharge rate and response pattern with those of higher neurons indicates that the response to dynamic transients is facilitated, while the response to steady temperature states is largely filtered out. This suggests that the main function of this system is the localization of moving thermal objects. The reported sensitivity increase in the medulla as compared to the periphery could be due to synchronous input from converging fibres. The absence of inhibitory areas implies the absence of inhibitory input from the periphery, which is a feature peculiar to this system. A schema of neuronal connections is proposed based on the overlapping and enlargement of receptive areas of CNS neurons.